In the hugely competitive market of charity giving, why is it that retirement for overworked donkeys, The Donkey Sanctuary, ranks higher than, to pick an example at random, a charity devoted to a disease that is the third leading cause of death in the UK, The Stroke Association? And why does the Royal National Lifeboat Institution come fifth on the current list of voluntary income for charities in the UK, an anomaly among the big players of cancer, children and animals?

The outpouring of generosity around the tragic death of a charity runner in this year's London Marathon hints at the multitude of financial, psychological and historical factors at play when people decide how and where they donate money.

Claire Squires was one of a number of individuals raising money for the Samaritans, the charity that provides 24-hour emotional support for people who are in distress, including those who are suicidal. She collapsed and died a mile from the finish line. Since her death donations to her Just Giving page have increased at an unprecedented rate. When I started writing this article the total was around £330,000. When I finished, a couple of hours later, it was more than £400,000 and has already attracted the biggest number of donations in a single day since Just Giving was launched 12 years ago.

First, and perhaps most importantly, Squires is an individual – named, pictured, we know her motivations for doing what she did. Her mother has been a Samaritans volunteer for more than 24 years. We relate to her far more intimately than to any amount of statistics about the good works of the charity. The phenomenon is something many charities have adopted in their marketing for donations – the specific details of an individual are demonstrably more compelling than generalities.

Second, we have the "bystander effect" where if other people are standing around doing nothing it's easier for us to justify doing nothing. In this case it's the bystander effect in reverse – lots of other people are doing it and there's a lot of publicity about it and it makes it easier to join in.

Another reason it's easier to join in is the advent of internet fundraising sites. It takes far less effort to go to click the link to a widely publicised Just Giving page and donate money than it would to find out the details of the charity behind it and locate ways to donate to it directly. You can see everyone else who has made a donation, how much they've given and a running total of the money raised.

Above all, this is not what's known in fundraising circles as "a guilt ask". Charity donors are apparently becoming far more likely to respond negatively to requests for generosity based on the guilt trip, the bad news culture, and are becoming far more savvy about being given information and following up what the money is used for. Nobody is exhorting anyone to give money to the charity Squires supported. This may, in fact, be an important factor for quite a number of the people who now are doing so.

Which brings us back to the donkeys and the strokes. This specific example of collective generosity is the exception rather than the rule. Charities are chasing what is at best a level of donation that is remaining static or, more likely, shrinking as a result of the economic climate. There are more than 150,000 charities pursuing what is effectively a finite amount of money, more than three-quarters of which receive no government funding.

Income from individual donors remains the single most important source of revenue for the entire voluntary sector in the UK. In this market the aim is to "recruit" and "retain" as many long-term "regular givers" as possible, with accusations already being made by smaller organisations that the big players are using their position to squeeze out competition. Already about 0.5% of the UK's charities receive 54% of the entire voluntary sector's income.

It's not just a matter of size being important. Some causes seem to be inherently more appealing than others. Medical charities top the donation league with cancer topping the conditions, but within that there's a definite hierarchy. We are, it seems, far more willing to relate to breasts than to colons, at least in terms of where we chose to donate money, despite them both being in the top three cancers in the UK. Children garner more money than old people despite the vulnerability of both.

Ultimately the pattern of a society's charitable giving reflects that society's composition, interests and concerns. Most of the prized "regular givers" are older people. As that demographic changes so too will patterns of giving. But cynical as I may be on a personal level about the kind of charities we favour over others, I am still positively surprised to be part of a society that regards giving as important. According to the Charities Aid Foundation's World Giving Index, the UK is fifth overall scored on money and time donated, second overall in money alone.

As a volunteer who gives time on a weekly basis to a charity, I'd like to see us rise from 32 in the world on that ranking – but that's another story.